Method and apparatus for edge sealing corrugated paperboard

ABSTRACT

A corrugated core paperboard in which the edges of the the core convolutions are sealed by method and apparatus for preparing a film of thermoplastic material along the board edges; the method including the steps of feeding the board along a path, passing the edge of the board through an applicator and applying a film of thermoplastic material to the board edge as the edge passes through the applicator; the apparatus including driven belts between which the board is fed in a path, an applicator along the edge of the path and means for feeding thermoplastic material to the applicator.

United States Patent Frappier et al.

METHOD AND APPARATUS FOR EDGE SEALING CORRUGATED PAPERBOARD Inventors:Robert H. Frappier, Agawam,

Mass.; Hal W. Tanner, Jr., Georgetown, S.C.; William O. Wagers,Massillon, Ohio International Paper Company, New York, N.Y.

Filed: Sept. 30, 1971 Appl. No.: 185,323

Related US. Application Data Division of Ser. No. 851,973, Aug. 21,abandoned.

Assignee:

US. Cl 117/44, 156/196, 156/198, 156/210, 161/99, 161/108, 161/133,161/134, 161/147, 161/149 Int. Cl D21b l/10 7 Field ofSearch ..93/36 MM,36 PC, 36 R; 117/43,44; 118/44; 156/196, 198,210;161/43',44,99,108,133,134,147,149

References Cited UNlTED STATES PATENTS 2/1971 wagers et al. 161/102 Feb.5, 1974 2,293,747 8/1942 Koch 117/43 X 2,694,648 11/1954 2,742,7244/1956 3,107,586 10/1963 3,406,052 10/1968 3,540,354 11/1970 Tachibanaet al. 93/1 Primary ExaminerWilliam D. Martin Assistant Examiner M. R.Lusignan [57] ABSTRACT A corrugated core paperboard in which the edgesof the the core convolutions are sealed by method and apparatus forpreparing a film of thermoplastic material along the board edges; themethod including the steps of feeding the board along a path, passingthe edge of the board through an applicator and applying a film ofthermoplastic material to the board edge as the edge passes through theapplicator; the apparatus including driven belts between which the boardis fed in a path, an applicator along the edge of the path and means forfeeding thermoplastic material to the applicator.

5 Claims, 18 Drawing Figures PAIENIEB FEB SHEET 3 OF 6 PATENTEU FEB 74SHEEI t 05 6 IIIIII NR QWN v PMENTEDFEB SISH MISBFG rrl/ I I II IIPAIENIEQ FEB 5:974

sum 6 or 6 CORRUGATED PAPERBOARD CROSS REFERENCE TO RELATED APPLICATIONThis application is a divisional application of application Ser. No.851,973, filed Aug. 21, 1969, in the name of the present inventors, andnow abandoned.

This invention relates to corrugated core paperboard and, moreparticularly, to such paperboard in which one or more of the edges ofthe corrugated core are closed or sealed and to methods and apparatusfor the manufacture thereof.

Corrugated core paperboard, having a corrugated core with facing sheetsadhesively secured to the opposite faces thereof, is used for manypurposes including packages and containers for a wide variety ofproducts. The convolutions of the corrugated core space the facingsheets and, at the opposite edges of the board, form openings or spacesintowhich dirt, liquid, foreign matter or the material packaged in thecontainer might flow. Such dirt, liquid, foreign matter, or the like,when it flows into the convolution opening, not only detracts from theappearance and sanitary condition of the container but, in manyinstances, weakens the core and facing sheets and the adhensive bondinterconnecting such core and facing sheets. Because thecorrugate'd corematerial and inner surfaces of the facing sheets are, in most instances,relatively absorbent, the board is especially weakened when the materialflowing into the openings or spaces is liquid.

I Various attempts have, heretofore been made to close or seal the open,fluted edges of corrugated paperboard. In one such arrangement, theopen, fluted edge of the corrugated paperboard is passed through anextruder and a U or V-shaped bead of thermoplastic material is extrudedonto the edge. Such'a U or V- shaped bead, extruded onto the edge,thickens the edge of the paperboard which, in some operations, mightinterfere with further processing, for example, printing, of thepaperboard or, in other-operations, might become crushed infurther'processing. M0reover,'extruding atherrnoplastic bead along theedge substantially increasesth'e cost of the corrugated paperboard.

In'the instant invention, the, open, flutededg'e of corrugatedpaperboardis sealed by forming a thin film of thermoplastic materialover the flute openings and edges of the facing sheets. This isaccomplished by passing the edge of the board through an applicator and,as the edge of the paper-board passes through the applicator, coatingthe edge with, the thermoplastic material. Preferably, only enoughthermoplastic material is fed to the applicator to form a thin filmalong the edge. Such film need not be continuous so long as it issubstantially impervious to liquid, dirt and the like. The thermoplasticmaterial is applied without pressure and without an excess of thematerial. Thus, the excess of the thermoplastic material is drained awayfrom the applicator.

In the preferred embodiment of the instant invention, the open, flutededge of corrugated paperboard is toward the center of the board at theedge and while the edge is still in the applicator, a coating ofthermoplastic material is applied to the edge to form a thin film alongthe edge between the rolled ends of the facing sheets. The thin film,thus formed, seals the flute opening and edges of the facing sheets atthe board edge.

The invention will be more fully understood from the followingdescription and appended drawings in which:

FIG. 1 is a fragmentary perspective view of a corrugated paperboardhaving anopen, untreated edge;

FIG. 2 is a view, similar to FIG. 1, with a portion of one facing sheetbroken away and showing the edge of the board as sealed in oneembodiment of the invention;

- FIG. 3is a view, similar to FIG. 2 and showing the ends of the facingsheets, at the board edge, rolled inwardly, toward each other, showingthe edge of the board as sealed in the preferred embodiment of theinvention; I

FIG. 4 is a side elevational view of the machine of the instantinvention for sealing the edges of corrugated core paperboard;

FIG. 5 is a top plan view of the machine of FIG. 4;

. corrugated core paperboard and the applicator, showpassed through anapplicator and the ends of the facing sheets are rolled inwardly, towardeach other, thereby partially closing the corrugated core flute openingswith the rolled ends of the facing sheets at the edge of the board. Withthe facing sheet ends rolled inwardly ing the edge sealed in accordancewith one embodimerit of the invention;

' FIG. 11 is a view of the treated, corrugated board showing the edgesealed in'accordance with one embodiment of the invention;

FIG. 12 is a view, similar to FIG. 10 and showing the edge of the boardtreated in accordance with the preferred embodiment and the applicator;t

FIG. 13 is a view, similar to FIG. 12, but without the applicator,showing the corrugated core paperboard treated in accordance with thepreferred embodiment;

FIGS. 14 and 15 are vertical sections taken through the machine at lines14-14, 15- 15, respectively, FIG.

FIG. 17 is an enlarged detail taken at 17-17, FIG.

16; and a FIG. 18 is an enlarged detail taken at line 18-18,

FIG. 14.

Referring to FIGS. 1 to 3, the corrugated paperboard includes facingsheets 2, 4, adhesively secured to the opposite sides of corrugated core6, in the customary manner, such as on a conventional corrugatedpaperboard machine. The edge, or edges, of the corrugated paperboard tobeclosed are preferably trimmed, if necessary, so that, at each edge tobe closed, the ends of facing sheets 2, 4 and core 6- are substantiallyflush or FIG. 16 is a cross section taken at lines 16-16, FIG.

in substantial alignment with each other. The edge, or edges, so trimmedareadvanced longitudinally of the edge, through an applicator. Facingsheets 2, 4, as the corrugated core paperboard advances through theapplicator, engage corresponding walls of the applicator and form a sealtherewith. The inner wall of theapplicator is spaced outwardly, joiningthe applicator walls in engagement with the paperboard facing sheets, isspaced from the edge of the paperboard and a supply of liquid,thermoplastic material, for example, wax, compatible with the edge ofthe paperboard is supplied to the space, in the applicator, at the edgeof the board. As will be described later herein, such space in theapplicator into which such thermoplastic material is fed, is providedwith a drain so that any excess thermoplastic material in such spacewill drain out of the applicator.

As the open, corrugated edge, of the paperboard passes through theliquid, thermoplastic material in the applicator, the edge of theboardpicks up a thin film of the thermoplastic material which, upon setting,forms the thin, flexible film 8 along the edges of facing sheets 2, 4,corrugated core 6 and over the ends of the convolution openings betweencore 6 and facing sheets 2, 4. In the preferred embodiment of theinvention, the applicator, in addition to applying the thermoplasticmaterial to the edge of the board, rolls edge 10 of sheet '2 and edge 12of sheet 4 inwardly, toward each other.

Thus, in the embodiment of FIG. 3, the corrugated core edge is partiallyclosed by the rolled in edges 10, 12 of facing sheets 2, 4. The edge ofthe board, between rolled edges 10, 12 of facing sheet 2, 4, is sealedby thermoplastic film 8.

In the apparatus and method of the instant invention, the edge, oredges, of the corrugated board to be sealed are fed through anapplicator by feeding a sheet of corrugated paperboard along a path in adirection transverse to the direction of the convolutions of thecorrugated core. An applicator is positioned at the side of. the pathalong which the open edge, tobe sealed, will pass. When both edges ofthe corrugated paperboard are to besealed, applicators are positionedalong both path lines.

The applicators are positioned along the path sides so that the boardedge to be sealed passes into, and

through, the applicator. The top and bottom facing sheets of the boardengage the upper and lower walls, respectively, of-the applicator slot.ln the preferred embodiment of the invention, the applicators arepositioned so that the edges of the top and bottom facing sheets of theboard engage the arcuate wall of the applicator joining the top andbottom applicator walls. By engaging such arcuate applicator wall, theedges of the corrugated core, paperboard, top and bottom facing sheets,at the corrugated board edge, are rolled inwardly, toward each other,partially closing the open edge of the corrugated core. Thermoplasticmaterial is pickedup by the board edge as it passes through theapplicator, and the thermoplastic material is spread along I the openingbetween theedges of the facingsheets. Upon setting, the thermoplasticmaterial forms a thin, flexible film 8 along the edges of facing sheets2, 4, corrugated core 6 and between the facing sheets, sealing the openend of the core convolutions between the facing sheet edges.

Referring to FIGS. 4 to 18, the apparatus of the instant inventionincludes a table, generally designated bearings 54, 56, 58, 60, fixed tosupports 40, 42 and,

intermediate their ends, are threaded at 62, 64 (FIG. 14), for purposeshereinafter described. Sprocket 66 and crank handle 68 are keyed to oneend of shaft 50 and double sprocket 70 is keyed to the end of shaft 52.

Referring to FIGS.v 4 and 15, shafts 72, 74 are mounted, at theiropposite ends, in bearings 76, 78, 80, 82, fixed to supports 44, 46,.respectively, and are threaded, intermediate their ends, at 84, 86, forpurposes later described. Sprocket 88 is keyed to the end of shaft 72and double'sprocket 90 is keyed to the end of shaft 74. Sprocket 66 isconnected to one sprocket of double sprocket 70 by chain 92 (FIGS. 4 and14) andthe other sprocket of double sprocket 70 is connected by chain 94to one of the sprockets of double sprocket 90. The other sprocket ofdouble sprocket 90 is connected by chain 96 to sprocket 88. By turningcrank 68, shafts 50, 52, 72, 74 are rotated in their respective bearingsin supports 40, 42, 44, 46 through the inter connection of chains 92,94, 96, sprocket 66, 70,

' 88 and 90 keyed, respectively, to shafts 50, 52,72, 74.

I As best shown in FIG. 14, shaft 100, having longitudinally. extendingkeyway 102, and shaft 104, having a longitudinally extending keyway 106,are mounted, at their opposite ends, for rotation in bearings 108, 110,

112, 114 fixed to-supports 40, 42, respectively.

Sprocket 116 is keyed to shaft 102 and sprockets 118, 120 are keyed toshaft 104. Idler sprocket 122 is mounted for rotation on bracket 124fixed tomember 126 of frame 34. Referring to FIG. 18, sprockets 1116,118, 122, are interconnected by chain 130. Sprocket 120 is connected bychain 132 to variable speed drive driving roll is held in fixedpositionon shaft 100 by thrust bearings 150, 152, mounted in plates 154,156 attached by bolts to plates 158, 160, fastened, as by, for example,welding, to block 161, mounted, by bearings 162 and 164 and collars 166,168, fixed, by set screws 170, 172, to shaft 50. Belt driving roll 142is held in fixed position on shaft 100 by thrust bearings 180, 182mounted in plates 184, 186'bolted to plates 188, 190, respectively,fastened, as by, for example, welding, to block 192 threaded by sleeve194 to threads 62 on shaft 50.

Belt driving roll 144 is held in fixed position on shaft 104 by thrustbearing 200, 202 mounted in plates 204, 206 bolted to plates 208, 210fastened, as by, for example, welding to block 212 mounted in fixedposition on shaft 52 by collars 214, 216. Belt driving roll 146 is heldin fixed position on shaft 104 by thrust bearing Referring to FIG. 15,plates 158, 160, 188, 190, 208, 210 and 228, 230 (FIGS. 14 and 15)extend longitudinally of frame 34. Belt roll 240 is mounted for rotationon plates 158, 160 by bearing 242, 244 mounted on plates 246, 248 boltedto plate 158, 160. Plates 158, 160 are fastened, as by, for example,welding, to block 250 mounted in fixed position by collars 252, 254 onshaft 72. Extension 256 of block 250 extends over transverse framemember 260 of frame 34 and has adjusting set screw 262 for adjusting thevertical position of block 250 and belt roll 240.

Belt roll 264 is mounted for rotation on plates 188, 190 by bearing 266,268 mounted on plates 270, 272 bolted to plates 188, 190. Plates 188,190 are fastened, as by, for example, welding, to block 274 threaded onthreads 84 of shaft 72. Roller 278 on extension 276 of block 274 issupported on the upper surface of transverse frame member 260.

Belt roll 280 is mounted for rotation on plates 208, 210 by bearings282, 284 mounted on plates 286, 288

bolted to plates 208, 210. Plates 208, 210 are fastened,

as by, for example, welding, to block 290 mounted in fixed position onshaft 74 by collars 292, 294. Block 290 rests on transverse frame member296 of frame 34.

Belt roll 300 is'mounted for rotation in plates 228, 230 by bearing 302,304 mounted in plates 306, 308 bolted to plates 228, 230. Plates 228,230 are fastened, as by, for example, welding,-to block 310 threadedonto threads 86 of shaft 74 and roller 312 of block 310 rests ontransverse frame member 296.

Endless belt 314 extends around belt driving roll 140,

and belt roll 240. Endless belt 316 extends around belt driving roll 142and belt roll 164. Endless belt 318 extends around belt driving roll 144and belt roll 280.

Endless belt 320 extends around belt driving roll 146 318 with opposedbelts 316, 320 may be adjusted for wider or narrower sheets.

' 'In the illustrated embodiment of the apparatus of the instantinvention, the open, or fluted edges of the core of the corrugatedpaperboard, at the opposite edges of the board, as the board is fedthrough the apparatus, pass through an applicator where a thermoplasticmaterial, in the form of a thin film, is applied to the edge of the coreto seal the core. This is accomplished at one edge or at both edgessimultaneously by components located along the edge of the paperboardfeed path as the board is fed between belts 314, 318 and 316, 320. Suchcomponents, where both edges are to be sealed simultaneously, aremounted on plates 190, 230 and 158, 200 and are movable with suchplatesas the plates and belts are adjusted with crank 68 to accommodatewider or narrower boards. Because such thermoplastic applicatorcomponents on the respective plates are identical, the componentsattached to plates 190, 230 are described, it being understood,'however, that in the embodiment of the apparatus for sealing both edges,simultaneously, such components are duplicated on plates 158, 200.

Referring to FIGS. 6, 7, 8, 16 and 17, plates 188, 190 are held infixed, parallel, spaced position by spacers 330, 332, 334 and plates228, 230 are held in fixed, parallel, spaced position by spacers 336,338, 340. R01- ler 342, mounted at their opposite ends in plates 188,

mounted on plate 190 by bolts 354. The lower edge of guide plate 350 isadjusted by slots 352 and bolts 354 to engage and guide the uppersurface of the corrugated core paperboard as the board is fed throughthe apparatus. v

Brackets 360, 362 are mounted on plate 230 by bolts 365, 366, and 368,370, respectively, and 'plate 364 is fastened to brackets 360, 362 asby, for example, welding. With particular reference to FIGS. 6, and 8,adjustable bracket- 374 is mounted on plate 364 and is held in adjustedposition thereon by bolt 376 in slot 378 in plate 364 and adjustingscrew 380 threaded in member 382 fixed as by, for example, 'welding toplate 364. Applicator 384 is mounted on adjustable bracket 374 by bolt386 and locking nut 388.

Referring to FIGS. 6, 7 and 8, applicator 384 has, at

itsface, facing the corrugated board path, a slot formed I by upper andlower walls 390, 392 and inner curved wall 394 joining walls 390, 392.At the entrance end of the slot, at the right hand end, FIG. 7, walls390, 392 flair away from each other, forming an enlarged entrance forthe board edge. Thermoplastic fluid passage 396 opens into the slot andis connected by passage 398 and conduit 400 to a thermoplastic materialsource, not shown. The lowerwall 392 of the applicatorslot has a drain402 which, as later described, allows excess thermoplastic material fedto the slot through passage 3% to drain out of and away from theapplicator slot. Such material, draining out of and away from the slotcan, of course, be recycled, reheated and refed to the applicator slot.v

In the operation of the apparatusof the instant invention, corrugatedpaperboard, with its edge or edges to be closed'trimmed so that theedges are substantially square and I with the facing sheet edgessubstantially flush with the ends of the core convolutions, is fedthrough the apparatus in the direction of the arrow, FIG. 5, andtransverse to the direction of the convolutions of the core of thecorrugated paperboard. The

board may be fed as a continuous strip from the end of With motor 136running, and through variable speed i i drive 134, driving belts 314,318, 316, 320, the belts feed the corrugated paperboard through themachine. Plates 350 are adjusted so that-the upper facing sheet surfaceslides along the bottom surface of the plates.

The feed of thermoplastic material through conduit 400 and passages 398,396 is adjusted so that, at the open end of passage 396, a constantsupply of thermoplastic material is maintained as the edge of thecorrugated board is passing through the applicator. This is importantbecause, if too little material is supplied, the required film ofthermoplastic material will not be formed along the edge of the board.If, on the other hand, excess material is supplied, such excessmaterial, over and above that deposited as a film along the edge of theboard, will drain off through drain 402. In any event, the thermoplasticmaterial is coated onto, and .not pressure forced into, the board edge.Thus, there is formed along the edge, a thin film and the thermoplasticmaterial is not forced into the open corrugated ends.

The corrugated paperboard edge, as it passes the end of passage 396,picks up a film of thermoplastic material. Depending upon the speed atwhich the paperboard is fed, the thickness of the board, the size of theopening at the end of passage 396, such film along the edge of the boardmay be formed with one application or a plurality of applications inspaced alignment.

The applicators may be adjusted so that the edges of the upper and lowerfacing sheets engage upper and lower walls 390, 392 without engaginginner curved wall 394 or so that the edges of the facing sheets engagethe inner curved wall. When inner curved wall 394 is not engaged, thethermoplastic material is deposited between the facing sheets and formsfilm 8, FIG. 2, therebetween. When on the other hand inner curved wall394 is engaged, the edges l0, 12 of facing sheets '2, 4, FIG. 3, aredeflectedor curved inwardly, toward each other, as the thermoplasticmaterial is being deposited. Thus, in the embodiment of the invention,as shown in FIG. 3, the space between the facing sheet edges issubstantially reduced and the amount of thermoplastic material depositedand forming film 8, is decreased.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible.'

What is claimed is: I g

1. In an apparatus for edge sealing a corrugated paperboard with athermoplastic film, wherein the paperboard comprises a corrugated coreand facing sheets secured to the core, which includes means foradvancing the paperboard in a direction transverse to the convolutionsof the core, and applicator means for applying a plastic film across anedge of the paperboard, an improved applicator means comprising: r

1 a stationary block having a slot in a surface thereof which extends ina direction substantially parallel to the direction in which thepaperboard is advanc- I ing, the block being disposed along thedirection of advance so that an edge of the paperboard which istransverse to the convolutions of the core passes through the slot andis enclosed within the slot, said' 8 b. a flat lower wall slidablyengaging the outer surface of the lower facing sheet of the paperboard;

c. an intermediate wall inward of the block surface and adjacent theedge of the paperboard passing through the slot, said intermediate wallconnecting said upper and lower wall and having therein a chambercommunicating with the slot; and d. a flared enlargement at the end ofthe slot at which the paperboard enters for guiding the paperboard intothe slot; and

2. means forinjecting thermoplastic through the chamber in theintermediate wall into the slot against the moving edge of thepaperboard passing through the slot and directly against the exposedcorrugated core of the paperboard to seal the edge with a thermoplasticfilm formed by said injection,

. said film extending only across the edge of the paperboard.

2. The improved applicator of claim 1 wherein the lower wall of the slotcontains a drain for draining off and recovering excess thermoplasticinjected into the slot.

3. The improved applicator of claim 1 where the intermediate wall isarcuate to thereby cause the facing sheets upon engaging the arcuateintermediate wall to bend inwardly-toward the corrugated core prior tosaid thermoplastic being applied to the edge.

4. In a method for sealing an edge of a corrugated paperboard with athermoplastic film, wherein the paperboard comprises a corrugated coreand facing sheets secured to the core, which includes advancing thepaperboard in a direction transverse to the convolutions of the core andpast a thermoplastic applicator, and applying a thermoplastic film to anedge of the paperboard, the improvement which comprises:

l. passing an edge of the paperboard which is transverse to theconvolutions of the core through a stationary slot which extends in adirection substantially parallel to the direction in which thepaperboard is advancing, the edge of the paperboard being enclosedwithin the slot, said slot being formed by; l i I a. a flat'upper wallslidablyengaging the outer surface of the upper facing sheet of thepaperboard;

face of the lower facing sheet of the paperboard;

c. an intermediate wall inward of the block surface and adjacent theedge of the paperboard passing through the slot, said intermediate wallconnecting the upper and lower wall and having therein a chambercommunicating with the slot; and

d. a flared enlargement at'the end of the slot at which the paperboardenters for guiding the paperboard into the slot; and

2. injecing settable fluid thermoplastic material through the chamber inthe intermediate wall into the slot against the moving edge of thepaperboard passing through the slot and directly againstthe exposedcorrugated core of the paperboard to seal the edgewith a thermoplasticfilm formed by said injection, said film extending only across the edgeof the paperboard, and setting the thermoplastic material.

5. The improved method of claim 4 wherein the in-' ing sheets to bendinwardly toward the corrugated core prior to applying the thermoplasticof the paperboard.

material to the edge P0-1050 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,790,399 Dated Februarv 3 ILQ'TLL Inventor(s) ertH. Frappier, et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 5-, line 3A, "16A" should be 26 Column 8, line 59, injecing"should be injecting Signed and sealed this 21st day of May l9'Z14..

(SEAL) Attest:

EDWARD I'LFLETCHERJR. I C. MARSHALL DANN Attesting Officer Commissionerof Patents

2. means for injecting thermoplastic through the chamber in theintermediate wall into the slot against the moving edge of thepaperboard passing through the slot and directly against the exposedcorrugated core of the paperboard to seal the edge with a thermoplasticfilm formed by said injection, said film extending only across the edgeof the paperboard.
 2. The improved applicator of claim 1 wherein thelower wall of the slot contains a drain for draining off and recoveringexcess thermoplastic injected into the slot.
 2. injecing settable fluidthermoplastic material through the chamber in the intermediate wall intothe slot against the moving edge of the paperboard passing through theslot and directly against the exposed corrugated core of the paperboardto seal the edge with a thermoplastic film formed by said injection,said film extending only across the edge of the paperboard, and settingthe thermoplastic material.
 3. The improved applicator of claim 1 wherethe intermediate wall is arcuate to thereby cause the facing sheets uponengaging the arcuate intermediate wall to bend inwardly toward thecorrugated core prior to said thermoplastic being applied to the edge.4. In a method for sealing an edge of a corrugated paperboard with athermoplastic film, wherein the paperboard comprises a corrugated coreand facing sheets secured to the core, which includes advancing thepaperboard in a direction transverse to the convolutions of the core andpast a thermoplastic applicator, and applying a thermoplastic film to anedge of the paperboard, the improvement which comprises:
 5. The improvedmethod of claim 4 wherein the intermediate wall is arcuate, includingthe further step of causing the facing sheets of the paperboard toengage the arcuate intermediate wall thereby causing the facing sheetsto bend inwardly toward the corrugated core prior to applying thethermoplastic material to the edge of the paperboard.